1. Technical Field
The invention relates to the growth of epitaxial layers on semiconductor integrated circuits such as charge coupled devices, and in particular to molecular beam epitaxy of delta-doped layers onto the back surface of a already-fabricated backside-illuminated CCD.
2. Background Art
Co-pending U.S. patent application Ser. No. 07/905,012 filed Jun. 26, 1992 by Michael E. Hoenk et al. entitled "GROWTH OF DELTA-DOPED LAYERS ON SILICON CCDs FOR ENHANCED ULTRAVIOLET RESPONSE" and assigned to the present assignee discloses a process for enhancing the ultraviolet quantum efficiency of a backside-illuminated silicon CCD by growing a delta-doped epitaxial silicon layer on the back surface thereof, by molecular beam epitaxy. Generally, performing molecular beam epitaxy on an already-fabricated integrated circuit such as a CCD is problematic, as will be described now.
In the typical molecular beam epitaxy process, the work piece, a silicon wafer,.is held by a sample holder in a molybdenum heater block using indium as a bonding agent. Typically, the molecular beam epitaxy process is performed prior to the deposition of metal or aluminum conductive layers, so that the process may be carried out at high temperatures without destroying the integrated circuits formed on the silicon wafer. However, if molecular beam epitaxy is to be performed on a finished integrated circuit (such as a CCD), a number of seemingly insurmountable problems are encountered.
For example, existing sample holder designs are not compatible with the requirements for MBE growth on pre-processed devices. Mounting a CCD to a molybdenum block using indium would destroy the device, and the dimensions of charge-coupled devices are incompatible with standard indium-free sample holders. Changing the dimensions of an indium-free holder may not be suitable, because of possible damage caused by locally high temperatures where molybdenum contacts the substrate. Temperatures in excess of 500 Celsius will destroy the aluminum contacts of a CCD, and heating the CCD to temperatures in the 400-500 degree Celsius range must be limited in duration. In addition, the ultra-high vacuum environment required for MBE is sensitive to contamination attributable to the sample holder, particularly in those cases in which the sample holder is formed of materials harmful to the MBE process.
What is needed is a way of holding a pre-fabricated integrated circuit such as a CCD imager chip or die without using impurities such as indium and without inducing stresses on the CCD die by localized heating (caused by incompatible heat capacities between the CCD die and the sample holder) and/or by mechanical stress (caused by incompatible thermal coefficients of expansion between the CCD die and the sample holder).